Downhole high resolution NMR sprectroscopy with polarization enhancement

ABSTRACT

An apparatus and method is discussed for characterizing a fluid sample downhole of aliphatic hydrocarbon compounds, aromatic hydrocarbon compound, or connate mud filtrates containing carbon-13 isotopes using an enhanced nuclear magnetic resonance (NMR) signal on a measurement-while-drilling device. To enhance the carbon-13 NMR signal these nuclei are being hyperpolarized. Either the Overhauser Effect (OE) or the Nuclear Overhauser Effect or optical pumping and the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) can serve as a mechanism for hyperpolarization of the carbon-13 nuclei.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of and claims priorityfrom U.S. patent application Ser. No. 10/404,408, entitled “A Method andApparatus for Combined NMR and Formation Testing For Assessing RelativePermeability with Formation Testing and Nuclear Magnetic ResonanceTesting”, by Georgi, et al filed on Apr. 1, 2003, which is incorporatedherein by reference in its entirety, and which is a continuation-in-partof and claims priority from U.S. patent application Ser. No. 09/910,209entitled “Apparatus and Method for In Situ Analysis of Formation Fluids”by Krueger et al., filed on Jul. 20, 2001, which is incorporated hereinby reference in its entirety. U.S. patent application Ser. No.10/404,408 claims priority from U.S. patent application Ser. No.60/369,268 entitled, “Combined NMR and Formation Testing” by Georgi etal. filed on Apr. 2, 2002, and claims priority from U.S. patentapplication Ser. No. 60/406,082 entitled, “A Method and Apparatus forCombined NMR and Formation Testing For Assessing Relative Permeabilitywith Formation Testing and Nuclear Magnetic Resonance Testing” by Georgiet al.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the field of downhole nuclearmagnetic resonance (NMR) investigation of wellbore fluids. Inparticular, the invention relates to methods for increasing NMR signalamplitudes in measurements of fluids in downhole environments.

2. Description of the Related Art

Performing measurements on fluid samples is desirable in many oilindustry applications. In the prior art, such measurements are typicallymade by bringing samples to the surface using sealed containers andsending the samples for laboratory measurements. A number of technicaland practical limitations are associated with this approach. Suchlimitations include the limited sample material extractable from alimited number of downhole locations. Also, samples undergo reversibleand irreversible changes as a result of the temperature and/or pressurechanges while being brought to the surface and during transportation.For example, gases come out of solution, waxes precipitate, andasphaltenes chemically recombine. Irreversible changes eliminate thepossibility of ever determining actual in situ fluid properties.Reversible changes are deleterious because they occur slowly andtherefore impact sample handling and measurement efficiency.Furthermore, since fluid analysis laboratories are frequently distantfrom the well site, there are substantial delays—often several weeks—inobtaining results. If a sample is for some reason corrupted or lostduring sampling, transportation, or measurement, there is no possibilityof returning to the well to replace it.

In view of the foregoing, various methods exist for performing downholemeasurements of petrophysical parameters of a geologic formation.Nuclear magnetic resonance (NMR) logging is among the most importantmethods which have been developed for a rapid determination of suchparameters, including formation porosity, composition of the formationfluid, the quantity of movable fluid, permeability and others. At leastin part this is due to the fact that NMR measurements areenvironmentally safe and are unaffected by variations in the matrixmineralogy. In a typical NMR run, a logging tool is lowered into adrilled borehole to measure properties of the geologic formation nearthe tool. The tool is pulled up at a known rate and measurements arecontinuously taken and recorded in a computer memory, so that at the endof the run a complete log is generated showing the properties of thegeologic formation along the length of the borehole. Alternatively, NMRlogging can be done while the borehole is being drilled.

NMR logging is based on the observation that when an assembly of nuclearmagnetic moments, such as those of hydrogen nuclei, are exposed to astatic magnetic field, they tend to align along the direction of themagnetic field, resulting in a bulk magnetization. The rate at whichequilibrium is established in such bulk magnetization upon provision ofa static magnetic field is characterized by the parameter T₁, known asthe spin-lattice relaxation time. Spin-lattice relaxation is caused byenergy transfer between the nuclei and the lattice. Another related andfrequently used NMR logging parameter is known as the spin-spinrelaxation time (also known as transverse relaxation time) T₂. Spin-spinrelaxation is caused by flip flop processes of neighboring spins. Thisresults in gradual loss of phase coherence of the magnetic moments andhence in a loss of macroscopic magnetization and hence in a loss of NMRsignal. Radiofrequency magnetic field bursts (known as “RF pulses”) areused to turn the macroscopic magnetization and to initiate NMRrelaxation (see below). It is possible by a succession of RF pulses togenerate so-called spin echoes. In fact it is possible to generate witha so-called CPMG sequence of pulses a sequence of spin echoes that decaywith the spin-spin relaxation time T₂. The NMR echo amplitude of thebegin of a CPMG sequence relates directly to the porosity of the earthformation (matrix independent), while both relaxation times provideindirect information about the composition and quantity of the formationfluid, the pore size distribution, and others.

It is not possible to generate a highly homogeneous magnetic fieldinside the earth formation. For this reason only the NMR signal strengthand relaxation can be derived from NMR in the earth formation. There isa need to obtain information about the composition of formation liquids.Formation liquid can be extracted from the formation and analyzed insidethe WL or LWD tool in an NMR spectrometer. In this NMR spectrometer theformation liquid sample is NMR-analyzed inside a magnet withcomparatively high magnetic field and high magnetic field homogeneity.These magnetic field properties allow chemical shift NMR analysis, notfeasible in the formation in situ. Details of this NMR analysis followin subsequent paragraphs.

The RF frequency f₀ needs to meet the NMR resonance condition: f₀=γB₀,where γ is the gyromagnetic ratio, a nuclear property specific to thekind of nucleus, and B₀ is the externally applied magnetic flux density.A single RF pulse tilts the macroscopic (nuclear) magnetization. Thehigher the pulse amplitude and the longer the pulse the more will theinitial equilibrium magnetization rotate away from the B₀ direction. Aso-called 90° or π/2 pulse tilts the magnetization from the direction ofB₀ to a direction perpendicular to B₀. After such a pulse the nuclearmagnetization precesses with the nuclear resonance frequency f₀=γB₀ in aplane perpendicular to the B₀ vector. The precessing macroscopicmagnetization induces a voltage in the NMR sensor coil, the freeinduction decay (FID). This NMR signal can be analyzed for frequencydistribution. This is done, e.g. by executing a Fourier transformation(FT) of the FID, which will yield a frequency spectrum. In general, anyknown method to convert time-domain data into a frequency spectrum canbe used as an alternative to a FT. If the B₀ field homogeneity issufficient, we will find that the frequency spectrum of an NMR signal ofa liquid possesses a fine structure. This is caused by the so-calledchemical shift that is caused by electrons. The chemical shift dependson the chemical environment of the nucleus. For this reason“Chemical-shift NMR” also called “High-resolution NMR” has been used fora very long time in laboratory NMR for chemical analysis. Alternatively,Continuous Wave NMR (CW NMR) may be used instead of the pulsed NMR justdescribed. CW NMR sweeps either the magnetic field or the RF frequencyover the NMR resonance region observing increased RF absorption at theNMR resonances. This way a frequency spectrum is directly acquiredwithout the need for a Fourier transform. (CW NMR got somewhat out offashion when the Fast Fourier Transform (FFT) algorithm and powerfuldigital processors became available.)

Nuclei most often used for Chemical-shift NMR are ¹H (protons) and ¹³C.Chemical shifts of ¹H are not more than 10 ppm of the NMR resonancefrequency of isolated protons. To make ¹H chemical shift NMR work arelative inhomogeneity of the external magnetic field B₀ of far lessthan 1 ppm is required. Carbon-13 NMR (¹³C NMR) chemical shifts aretypically at least an order of magnitude greater and hence require lessstringent magnet homogeneity. But ¹³C has a low natural abundance ofonly 1% of the total carbon content and a gyromagnetic ratio which is aquarter of that of hydrogen. This results for ¹³C in a NMR sensitivitythat is approximately 6000 times lower than the NMR sensitivity of ¹H(at the same B₀). Carbon-13 spectroscopy is especially useful indetermining the chemical composition of carbon-containing compounds and,as said before, requires not such a very homogeneous magnetic field as¹H spectroscopy.

Some uses of carbon-13 spectroscopy are discussed in prior art. U.S.Pat. No. 5,306,640, issued to Vinegar et al., discusses a method formore accurately determining in-situ oil and brine saturation in poroussamples using NMR. Vinegar '640 uses NMR methods for rapidnon-destructive analysis of sponge core and obtains information aboutoil composition and viscosity, which can be obtained simultaneously. Themethod differentiates between crude oil and water based onfrequency-resolved chemical shift NMR spectroscopy of the crude oil andwater in a porous medium. The patent of Vinegar '640 uses carbon-13 NMRspectroscopy and a weighted carbon density of the oil to determine avolume of oil.

The method of U.S. Pat. No. 6,111,409, issued to Edwards et al.,discusses a method of characterizing a fluid sample withdrawn from anearth formation. Edwards '409 discusses performing nuclear magneticresonance spin echo measurements on the fluid sample at a nuclearmagnetic resonant frequency of carbon-13. Amplitudes of the spin-echomeasurements are summed. The summed measurements are spectrallyanalyzed. The fluid is characterized by determining whether aromatichydrocarbons are present. This characterization is done by measuring anamplitude of the spectrally analyzed spin echo measurements at about 130parts per million frequency shift from the carbon-13 resonant frequency.The fluid is also characterized by determining whether aliphatichydrocarbons are present by measuring an amplitude of the spectrallyanalyzed spin echo measurements at about 30 parts per million frequencyshift.

Carbon-13 NMR signals are typically weak due to the low naturalabundance of this nucleus and the low polarizations attainable inthermal equilibrium at normal magnetic fields and temperatures downhole.On the other hand, a high-resolution ¹³C chemical shift NMR spectrum canbe powerful in analyzing the chemical composition of hydrocarbonsdownhole. There is a need for a method of enhancing NMR signals in adownhole environment. The present invention fulfills that need.

SUMMARY OF THE INVENTION

One embodiment of the present invention is an apparatus and method forcharacterizing a fluid sample obtained downhole using an enhancednuclear magnetic resonance (NMR) Carbon-13 signal. This isotope is foundin all hydrocarbons and connate formation fluids and in borehole mud,typical of the downhole environment. The ¹³C NMR signal strength is verymuch improved by polarization enhancement. The ¹³C nuclei are beinghyperpolarized beyond the thermal equilibrium polarization normallypossible in the applied static magnetic field. The apparatus of thepresent invention can be conveyed downhole on a wireline device or on ameasurement-while-drilling device. The apparatus comprises a sensordevice, a fluid inlet connected to the sensor device for obtaining afluid from the earth formation into the sensor, and a fluid dischargefor discharging the fluid sample from the sensor device into theborehole. In one embodiment an agent chamber, connected to the fluidinlet, injects its contents, typically a polarizing agent, into thefluid inlet.

In a first aspect the polarizing agent is responsive to electron spinresonance, i.e. it contains atoms or molecules with unpaired electrons.The mixture (here called the sample) of formation fluid and polarizingagent is transferred into the NMR/ESR probe that is in the magnet. Inthe magnet the sample is first magnetized to thermal equilibrium. Bysubjecting the sample to high frequency (HF), meeting the ESR resonancecondition, and making use of the Overhauser effect (OE), thepolarization of the ¹³C nuclei can be enhanced beyond equilibrium by ahyperpolarization factor of up to 2600 (theoretical maximum). Once the¹³C nuclei are hyperpolarized any known ¹³C measurement can be executedby radiating the appropriate RF pulse sequence or by performing a CW NMRmeasurement at the ¹³C resonance frequency. The amplitude of thereceived ¹³C signals will be enhanced by the hyperpolarization factor. Adescription of the Overhauser Effect and also of the Nuclear OverhauserEffect is found in the monograph of C. P. Slichter, “Principles ofMagnetic Resonance”, 3rd enlarged and updated edition 1990.

In a second aspect the phenomenon of the Nuclear Overhauser Effect (NOE)is used to generate the hyperpolarization of the ¹³C nuclei. The energydifference of the spin up and spin down states in ¹H is about 4 times ofthat of the energy difference of the two spin states in ¹³C. Analogousto the OE of the previous chapter the ¹H transition can be saturated(instead of the unbound electrons) by radiating RF at the ¹H resonancefrequency. The ¹H spin system can couple to the spin system of ¹³C. Theresult is an increase in the population difference between spin up andspin down states of the ¹³C system beyond the thermal equilibrium, i.e.the carbon nuclei are being hyperpolarized. Once the ¹³C ishyperpolarized the ¹³C NMR is executed with enhanced signal amplitudeand enhanced signal-to-noise ratio as described elsewhere in this patentapplication. The advantage of the described method is that hydrogen isnaturally present in any sample of formation fluid. In contrast to OE noextra polarization agent needs to be used.

In a third aspect of the invention the polarizing agent can be polarizedby optical pumping with circularly polarized light (most convenientlygenerated by a LASER) and making use of the Spin Induced NuclearOverhauser Effect (SPINOE) of which details can be found in Boyd M.Goodson, “Advances in Magnetic Resonance, Nuclear Magnetic Resonance ofLaser-Polarized Noble Gases in Molecules, Materials, and Organisms”,Journal of Magnetic Resonance, vol. 155, 157-216 (2002). The polarizingagent can be polarized in the agent chamber before being injected intothe fluid sample. Alternatively it may be possible to polarize thepolarizing agent after mixing with the formation fluid sample eitherstill outside the magnet or inside. Different variants of opticalpumping and SPINOE are needed depending on these alternatives (seebelow). Typically the polarizing agent is a noble gas with traces ofother gases. In one instance, the polarizing agent can be xenon withtraces of a vaporized alkali metal and nitrogen.

Characterizing the fluid sample typically involves obtaining a NMRsignal (FID) of the hyperpolarized ¹³C. Such NMR signals arise from anysubstances containing carbon nuclei, in particular from hydrocarbons.The carbon-13 signal is enhanced due to a process of polarizationtransfer between the nuclei of the polarized polarizing agent and thecarbon-13 atoms. A process known as a Spin Induced Nuclear OverhauserEffect (SPINOE) can serve as a mechanism for nuclear spin transfer.

An alternative method of characterizing the fluid sample involves highresolution (or chemical shift) NMR at the resonance frequency of thehyperpolarized agent, e.g. xenon.

The present invention is a method for characterizing a fluid samplewithdrawn from an earth formation. Nuclear magnetic resonancemeasurements are performed on fluid samples obtained downhole at amagnetic resonant frequency of typically carbon-13. These measurementsare transformed into a frequency spectrum, e.g. by Fourier transform orany other applicable method. The frequency spectrum is analyzed and thechemical composition of the fluid sample (as far as the moleculescontain carbon) is determined.

In a first embodiment the method fierter comprises measuring a magnitudeof a static magnetic field used to make the ¹³C NMR measurements andsuperimposing a selectable magnitude magnetic field on the staticmagnetic field to compensate for temperature induced changes in themagnitude of the static magnetic field. A magnetic field sensing device,e.g. hall sensor, is used to measure the magnetic flux density.Alternatively and preferably the ¹H NMR resonance of the fluid undertest may be used to measure and regulate the static magnetic field.

In a second embodiment the method further comprises measuring amagnitude of a static magnetic field used to make the ¹³C NMRmeasurements, but no selectable magnetic field is superimposed. Afteracquisition of the ¹³C NMR signals these signals will then becomefrequency corrected using the result of the magnetic field measurement.

The homogeneity of the static magnetic field may be optimized bysuperimposing a number of selectable magnetic field gradients. Themagnetic field homogeneity may be tested by analyzing the ¹H NMR signaleither in the time domain by testing the length of the ¹H FID or aftertransformation into a frequency spectrum by testing the width of theresonance line. A regulation algorithm varies the superimposed fields sothat the length of the ¹H FID is maximized or the resonance line widthis minimized or matches a predefined shape. Instead of pulsed ¹H NMR CW¹H NMR may be used for the magnetic field regulation.

The method can further comprise performing nuclear magnetic resonancespin echo amplitude measurements, e.g. using a CPMG sequence, at aresonant frequency of hydrogen nuclei, and determining a relaxation rateor a distribution of relaxation rates of the hydrogen nuclei.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood with reference to theaccompanying figures in which like numerals refer to like elements, andin which:

FIG. 1 (Prior Art) shows a measurement-while-drilling device suitablefor use with the current invention,

FIG. 2 shows a nuclear magnetic resonance (NMR) sensor according to theinvention disposed in a hydraulic isolation chamber,

FIG. 3 (Prior Art) shows an end view of the NMR sensor of the inventiondetailing the location of permanent magnets and antennas,

FIG. 4 (Prior Art) shows a functional block diagram of circuits used tomake NMR spectroscopy measurements using the NMR sensor of theinvention,

FIG. 5 (prior art) shows the principle of the Overhauser Effect (OE),

FIG. 6 a (Prior Art) shows the principle of polarizing an alkali atom byoptical pumping with circularly polarized light,

FIG. 6 b (Prior Art) shows polarization of xenon nuclei via collisionand spin exchange,

FIG. 7 (Prior Art) shows a pad-mounted tool comprising an NMR orresistivity device and formation testing probe,

FIGS. 8A-8C (Prior Art) show representative analyses for connate fluid,aromatic-based mud filtrate, and aliphatic-containing crude oil, usingthe method of the invention,

FIG. 9 (Prior Art) shows a timing diagram for NMR measurement sequencesmade using the apparatus of the invention,

FIG. 10 (Prior Art) shows a diagram of the processes of nuclear spinpolarization transfer,

FIG. 11 (Prior Art) shows a progression of the spin transferinteractions with the various degrees of polarization at each stage, and

FIGS. 12, 12A, 12B (Prior Art) show configurations of magnets, antennaand shield suitable for use in obtaining in situ measurements with thepresent invention,

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic diagram of a drilling system 10 with adrillstring 20 carrying a drilling assembly 90 (also referred to as thebottom hole assembly, or “BHA”) conveyed in a “wellbore” or “borehole”26 for drilling the wellbore. The drilling system 10 includes aconventional derrick 11 erected on a floor 12 which supports a rotarytable 14 that is rotated by a prime mover such as an electric motor (notshown) at a desired rotational speed. The drillstring 20 includes atubing such as a drill pipe 22 or a coiled-tubing extending downwardfrom the surface into the borehole 26. The drillstring 20 is pushed intothe wellbore 26 when a drill-pipe 22 is used as the tubing. Forcoiled-tubing applications, a tubing injector, such as an injector (notshown), however, is used to move the tubing from a source thereof, suchas a reel (not shown), to the wellbore 26. The drill bit 50 attached tothe end of the drillstring breaks up the geological formations when itis rotated to drill the borehole 26. If a drill pipe 22 is used, thedrillstring 20 is coupled to a drawworks 30 via a Kelly joint 21, swivel28, and line 29 through a pulley 23. During drilling operations, thedrawworks 30 is operated to control the weight on bit, which is animportant parameter that affects the rate of penetration. The operationof the drawworks is well known in the art and is thus not described indetail herein.

During drilling operations, a suitable drilling fluid 31 from a mud pit(source) 32 is circulated under pressure through a channel in thedrillstring 20 by a mud pump 34. The drilling fluid passes from the mudpump 34 into the drillstring 20 via a desurger (not shown), fluid line28 and Kelly joint 21. The drilling fluid 31 is discharged at theborehole bottom 51 through an opening in the drill bit 50. The drillingfluid 31 circulates uphole through the annular space 27 between thedrillstring 20 and the borehole 26 and returns to the mud pit 32 via areturn line 35. The drilling fluid acts to lubricate the drill bit 50and to carry borehole cutting or chips away from the drill bit 50. Asensor S₁ placed in the line 38 can provide information about the fluidflow rate. A surface torque sensor S₂ and a sensor S₃ associated withthe drillstring 20 respectively provide information about the torque androtational speed of the drillstring. Additionally, a sensor (not shown)associated with line 29 is used to provide the hook load of thedrillstring 20.

In one embodiment of the invention, the drill bit 50 is rotated by onlyrotating the drill pipe 22. In another embodiment of the invention, adownhole motor 55 (mud motor) is disposed in the drilling assembly 90to, rotate the drill bit 50 and the drill pipe 22 is rotated usually tosupplement the rotational power, if required, and to effect changes inthe drilling direction.

In one embodiment of FIG. 1, the mud motor 55 is coupled to the drillbit 50 via a drive shaft (not shown) disposed in a bearing assembly 57.The mud motor rotates the drill bit 50 when the drilling fluid 31 passesthrough the mud motor 55 under pressure. The bearing assembly 57supports the radial and axial forces of the drill bit. A stabilizer 58coupled to the bearing assembly 57 acts as a centralizer for thelowermost portion of the mud motor assembly.

In one embodiment of the invention, a drilling sensor module 59 isplaced near the drill bit 50. The drilling sensor module containssensors, circuitry and processing software and algorithms relating tothe dynamic drilling parameters. Such parameters can include bit bounce,stick-slip of the drilling assembly, backward rotation, torque, shocks,borehole and annulus pressure, acceleration measurements and othermeasurements of the drill bit condition. A suitable telemetry orcommunication sub 72 using, for example, two-way telemetry, is alsoprovided as illustrated in the drilling assembly 90. The drilling sensormodule processes the sensor information and transmits it to the surfacecontrol unit 40 via the telemetry system 72.

The communication sub 72, a power unit 78 and an MWD tool 79 are allconnected in tandem with the drillstring 20. Flex subs, for example, areused in connecting the MWD tool 79 in the drilling assembly 90. Suchsubs and tools form the bottom hole drilling assembly 90 between thedrillstring 20 and the drill bit 50. The drilling assembly 90 makesvarious measurements including the pulsed nuclear magnetic resonancemeasurements while the borehole 26 is being drilled. The communicationsub 72 obtains the signals and measurements and transfers the signals,using two-way telemetry, for example, to be processed on the surface.Alternatively, the signals can be processed using a downhole processorat a suitable location (not shown) in the drilling assembly 90.

The surface control unit or processor 40 also receives signals fromother downhole sensors and devices and signals from sensors S₁-S₃ andother sensors used in the system 10 and processes such signals accordingto programmed instructions provided to the surface control unit 40. Thesurface control unit 40 displays desired drilling parameters and otherinformation on a display/monitor 42 utilized by an operator to controlthe drilling operations. The surface control unit 40 can include acomputer or a microprocessor-based processing system, memory for storingprograms or models and data, a recorder for recording data, and otherperipherals. The control unit 40 can be adapted to activate alarms 44when certain unsafe or undesirable operating conditions occur.

FIG. 7 shows a pad mountable saturation-determining device of U.S.patent application Ser. No. 10/404,408, by Georgi, filed on Apr. 1,2003, having the same assignee as the present invention, and thecontents of which are incorporated herein by reference. Such asaturation-determining device can be, for example, an NMR or resistivitydevice and a formation testing tool mounted in an extensible probe andpad device on either a logging while drilling tool or a wire lineformation tester probe assembly, such as the Baker Atlas ReservoirCharacterization Instrument (RCI). RCI is detailed in U.S. Pat. No.5,303,775 by Michaels et al., which is incorporated by reference in itsentirety. A monitoring while drilling formation tester extensible probeassembly is detailed in U.S. patent application Ser. No. 09/910,209, byKrueger et al. cited above. In either configuration, wire line orlogging while drilling, the present invention provides for relativepermeability determination over time derived from formation and pressuredraw down testing over time combined with NMR or resistivity saturationmeasurements over time to determine relative permeability. As shown inFIG. 7, the resistivity/NMR saturation measurement is confined to anarea associated with a localized resistivity/NMR region of interest 716close to the borehole 732 within a few radii of the formation test toolprobe, that is, the radius of the internal passage or orifice providedfor ingress of formation fluid 730 and egress of completion fluidthrough the borehole wall 714 to the formation. The probe 710 extendsfrom the downhole tool to press and seal the probe face 719 against theborehole wall 714. Formation fluid is extracted from the boreholethrough the probe 710. Completion fluid is injected into the formationthrough the probe 710. The provision of adjacent formation testing andNMR equipment in the same downhole tool enables concurrent determinationof saturation level and absolute permeability with NMR (Coates-Timurequation) data and mobility data from draw down-buildup analysisperformed by the formation testing equipment.

The invention of U.S. patent application Ser. No. 10/404,408, by Georgi,also discusses injecting fluids comprised of hyperpolarized elements inthe formation. These hyper-polarized elements increase the NMR signaland response, thereby increasing the signal to noise ratio for the NMRmeasurements. In accordance with the present invention, the method ofU.S. patent application Ser. No. 10/404,408, by Georgi, can be performedby drawing the fluid of the borehole into a sensor device conveyed bythe drilling tool into the borehole and injecting these hyper-polarizedelement into the fluid upon entering the sensor.

Typical NMR methods used in a borehole employ a geometry in which thetesting device is inserted into the borehole and measures properties ofthe medium that surrounds it. For downhole testing of solid earthformations, this geometry holds many advantages. Borehole fluid samplescan also be drawn into a chamber situated within the drill tool andthereby be surrounded by NMR coils which can deliver RF pulses. Oneexample of this is seen in U.S. Pat. No. 6,111,409, issued to Edwards etal., having the same assignee as the present invention, and the contentsof which are incorporated by reference.

FIG. 2 shows a nuclear magnetic resonance (“NMR”) sensor 210 suitablefor use with the present invention which can be disposed at anyconvenient location along an hydraulic line. As fluid is withdrawn fromthe earth formation, it enters the sensor 210 through a fluid inlet 208in a pressure-sealed chamber 206. The pressure-sealed chamber 206 can bedisposed in a convenient location in the instrument housing tohydraulically isolate the fluid withdrawn from the earth formation.After NMR measurements are performed on the fluid in the chamber 206,continued operation of the pump (not shown) can cause the fluid to bemoved through a fluid discharge 212 in the chamber 206 into the pump foreventual disposal either into the wellbore or into a sample tank (notshown). It should be noted that the sensor 210 can also be located inthe pump discharge line if it is convenient for the system designer.

Agent chamber 220 is connected via flow channel 221 to fluid inlet 208.The contents of agent chamber 220 can be injected into the fluid influid inlet 208 prior to the entrance of the fluid into pressure-sealedchamber 206. Typically, agent chamber 220 contains a polarizing agentfor use in enhancing the NMR signal of the fluid sample using a methodof the present invention.

The sensor 210 can include permanent magnets 202A, 202B made from AlNiCoor Samarium-Cobalt or similar magnetic material having remanencemagnetization which is relatively stable with respect to temperature. Inthis embodiment of the invention, the magnets 202A, 202B can besurrounded by a substantially cylindrical flux closure or “yoke” 203.Each magnet 202A, 202B can have its own pole piece 204A, 204B on therespective face of each magnet directed towards the center of the sensor210. The magnets 202A, 202B, yoke 203, and pole pieces 204A, 204Bprovide a substantially homogeneous static magnetic field in the centerof the sensor 210. The direction of magnetization of the magnets 202A,202B is substantially perpendicular to the longitudinal axis of thesensor 210. Three radio frequency antennas 216A, 216B, 216C are disposedalong the axis of the sensor 210 in between the magnets 202A, 202B. Theantennas 216A, 216B, 216C are used for sequential NMR experiments on thefluid in the center of the sensor 210. The sensor 210 can include a Hallprobe 218 or similar device for measuring the magnitude of the staticmagnetic field induced by the magnets 202A, 202B so that the magnitudeand the homogeneity of the field can be adjusted for changes in thestrength of the 202A, 202B magnets with temperature, as will be furtherexplained.

The structure of the sensor 210 can be better understood by referring toan end view in FIG. 3. The magnets 202A, 202B are each polarized asshown by an arrow thereon, generally perpendicular to the longitudinalaxis of the sensor 210. The axial length of the sensor 210 should bemuch longer than the diameter of the region in the center of the sensor210 having substantially homogenous static magnetic field, so that NMRexperiments can be performed in different locations along the length ofthe sensor by each of the three antennas (216A, 216B, 216C in FIG. 3).Pole pieces 204A, 204B can be made of a high magnetic permeabilitymaterial such as soft iron or the like and can be attached to inner faceof each magnet 202A, 202B. The cylindrical yoke 203 can contact eachmagnet 202A, 202B on the face opposite the location of the pole pieces204A, 204B. The yoke 203 can be made from a high magnetic permeabilitymaterial similar to that used for the pole pieces 204A, 204B. Thecombination of yoke 203, pole pieces 204A, 204B and the magnets 202A,202B provides a substantially homogeneous static magnetic field betweenthe magnets 202A, 202B, the field polarized in the same direction as thepolarization direction of the magnets 202A, 202B. Shim coils 214A, 214Bcan be located in between the magnets 202A, 202B. The shim coils can beconnected to controllable direct current (DC) power sources to providesupplemental static magnetic fields for compensating changes in themagnetic field strength and homogeneity resulting from changes inambient temperature. An ensemble of shim coils and controllable DC powersupplies may be used to remove lower and higher order static fieldgradients in all three dimensions to optimize field homogeneity. Thelocation of the RF antennas with respect to the magnets 202A, 202B andshim coils 214A, 214B is shown generally at the uppermost antenna 216A.The antennas (216A, 216B, 216C in FIG. 3) can be wire coils wound sothat the RF magnetic field induced by the antennas is substantiallyparallel to the longitudinal axis of the sensor 210. This direction isalso perpendicular to the direction of the static magnetic field and istherefore suitable for performing NMR experiments. The arrangement shownin FIGS. 2 and 3 is only an example of arrangements of permanent magnetand antennas which have the requisite properties for conducting NMRexperiments in a fluid sample. Other arrangements of permanent magnetand antenna are possible, so the arrangement shown in FIGS. 2 and 3 isnot to be construed as a limitation on the invention. The principlerequirements for magnets and antennas is that the magnet induce asubstantially homogeneous magnetic field in the location of the fluid tobe analyzed, and that the antenna induces an RF magnetic field which isalso substantially homogeneous and perpendicular to the static magneticfield in the location of the fluid to be analyzed.

The arrangement of magnets, yokes and antennas shown in FIGS. 2 and 3provides a substantially homogeneous static magnetic field in acylindrical volume in the center of the sensor 210. If the cylindricalyoke 203 has an external diameter of about 6 cm as shown in FIG. 3, thehomogeneous static magnetic field will exist within a cylindrical volumeof about 1 cm. in diameter.

Operation of the sensor 210 can be better understood by referring toFIG. 4. The antennas 216A, 216B, 216C can be connected to a transceivercircuit 420 through a switching circuit 422. The transceiver circuit 420generally can include a radio frequency power source which generatescontrolled-duration pulses or RF power, and switching circuits forselectively connecting the selected antenna (216A, 216B or 216C) betweenthe RF source and a receiver circuit (not shown separately). Thereceiver circuit is for detecting voltages induced in the selectedantenna by nuclear magnetic resonance. Circuits suitable for thetransceiver 420 are described, for example, in U.S. Pat. No. 5,712,566issued to Taicher et al. The transceiver 420 also can include digitalsignal processing (“DSP”) circuits for performing certain calculationson the measurements.

Irrespective of the magnetic material from which they are made, themagnets (202A, 202B in FIG. 3) will to some degree have remanencemagnetization which is affected by the ambient temperature around themagnets. It is not at all unusual for well logging instruments to besubjected to a temperature range from 0° to 200° C. Since the NMRexperiments performed by the sensor (210 in FIG. 3) are intended to bemade in a homogeneous static magnetic field, the sensor 210 includesso-called “shim” coils 214A, 214B which selectively induce a magneticfield superimposed on the static magnetic field induced by the magnets(202A, 202B in FIG. 3). The intensity of the total static field can bemeasured by the Hall probe 218 or similar device, which can be connectedto a control circuit 424. The control circuit 424 applies a directcurrent the shim coils 214A, 214B, the magnitude of which is related tothe output of the Hall probe 218, so that the total magnitude of thestatic magnetic field in between the magnets 202A, 202B can bemaintained substantially constant. As is understood by those skilled inthe art, the magnetic resonant frequency of selectively RF-excitednuclei will depend on the magnitude of the static magnetic field inwhich they are polarized. By maintaining a substantially constant staticmagnetic field magnitude, the need to adjust the frequency of the RFmagnetic field for NMR experimentation can be reduced or eliminated. Theshim coils 214A, 214B and source 424 should be able to provide about 100Gauss superimposed field magnitude to be able compensate the staticmagnetic field for changes in remanence magnetization of the magnets(202A, 202B in FIG. 3). The amount of static field amplitude required tobe provided by the shim coils 214A, 214B will depend on the type ofmagnet material used for the magnets. Thermally more stable magnetmaterials such as AlNiCo or Samarium Cobalt will require smaller fieldadjustment using the shim coils 214A, 214B than other magnet materialssuch as ferrite. The resonance of a ¹H NMR measurement of the sampleunder test may be used instead of a Hall probe to measure the magnitudeof the static magnetic field and its homogeneity. An ensemble of shimcoils and controllable DC power supplies may be used to remove lower andhigher order static field gradients in all three dimensions to optimizefield homogeneity and to adjust the field strength for the chosennominal magnitude. Alternatively, instead of adjusting the magnitude ofthe static field, the ¹³C NMR reference frequency can be adjustedinstead.

Without hyperpolarization of the carbon-13 nuclei the ¹³C signal has avery low amplitude for this reason a measurement sequence according toFIG. 9 is needed, preferably these spin echo sequences are executedtwice at 30 ppm and 130 ppm from the nominal ¹³C resonance frequency toget optimal conditions for these resonances of carbon nuclei in aromaticand aliphatic hydrocarbons (ref. U.S. patent Carl Edwards??). Manyechoes may be accumulated on top of each other to increase thesignal-to-noise ratio.

With the employment of one or the other method of polarizationenhancement according to this invention the carbon signal amplitude maybe so high that the sequences of FIG. 9 are not needed but a single FIDis acquired after one RF pulse. This very simple pulse “sequence” may berepeated several times and the FIDs accumulated. The resultingaccumulated FID is then transformed by Fourier transform or anothermethod into a frequency spectrum. A person trained in the art ofchemical analysis by ¹³C NMR can without or with the help of a computerprogram interpret this spectrum and determine the chemical compositionof hydrocarbons in the formation fluid sample.

Overhauser predicted (see A. W. Overhauser, Phys. Rev 91, 476 (1953) andA. W. Overhauser, Phys. Rev. 92, 411 (1953)) that if one saturated theconduction electron spin resonance in a metal, the nuclear spins wouldbe polarized 1000-fold more strongly than their normal polarization inthe absence of the saturation. This is known as the Overhauser Effect(OE). The experiment was subsequently successfully performed by Carverand Slichter (see Carver et al. Phys. Rev. 92, 211 (1953) and Carver etal. Phys. Rev. 102, 975 (1956). Carver went on to show that this form ofpolarization was not restricted to a metal. In fact, it is not necessaryto transfer polarization between electrons and nuclei. One can transferpolarization from nucleus to nucleus. This is known as the NuclearOverhauser Effect (NOE). Both effects are used as part of thisinvention. In one aspect of the invention the hyperpolarization ofcarbon-13 is achieved by the Overhauser effect (OE) of which theprinciple is shown in FIG. 5. The upper part of FIG. 5 shows the systemin thermal equilibrium in a magnetic field. The higher energy states ofelectrons and ¹H nuclei are less populated. This follows from the factthat the population of energy levels follow a Boltzmann distribution. Asimilar result occurs for ¹³C nuclei (not shown in FIG. 5). The factorbe in the population of the electrons in spin down and spin up state isapprox. 10⁻³ while the equivalent factor δ¹³C is less than 10⁻⁶ at amagnetic flux density of 1 T and 20° C. Now referring to the lower partof FIG. 5, by radiating an RF field onto the electron spin resonance thepopulations of both electron energy levels have become equalized. Bycoupling of the electrons with the nuclei this increases the populationof the upper energy level of the nuclei very much and depopulates thelower level accordingly. The ¹H or ¹³C nuclei posses now a highpopulation difference, which is synonymous to a high polarization inexcess of the thermal equilibrium. In this embodiment of the invention,the ESR-active agent is stored in a tank in the tool. A small amount isadded to the fluid sample extracted from the formation. The sample isirradiated at the ESR resonance frequency to enhance the polarization ofthe nuclei under test and straight after that (less than the NMR T1) theNMR measurement (typically C¹³) is executed This method has been usedfor medical applications, where it is necessary that the ESR-activeagent be non-toxic. For NMR applications in a wellbore, the stringentrestriction of non-toxicity can be relaxed.

In a second aspect of this invention the hyperpolarization of ¹³C isachieved by the Nuclear Overhauser Effect (NOE) between ¹H and ¹³C. Thisis similar to the Overhauser Effect, but in this method the spinexchange is not between electrons and nuclei but between two kinds ofnuclei of which the one with the wider energy splitting(higher-gyromagnetic ratio, here ¹H) is being saturated. Under Certainconditions, depending on the relaxation times and concentrations of thekind of nuclei involved, the population difference of the nuclei withthe lower gyromagnetic ratio (here ¹³C) is increased and hence becomeshyperpolarized. Since hydrogen nuclei are already present in theformation fluid, it is not necessary to add any particular agent formaking measurements based on NOE for evaluation of a ¹³C signal. Allthat is necessary is to apply a (pulsed) RF field at the resonancefrequency of ¹H.

In a third aspect of the invention optical pumping is used to achievehyperpolarization. One such mechanism is shown in FIG. 6 a. This figureshows the S and P electron spin states of an alkali atom. No magneticfield is present which is why the two S spin states have equal energyand also the two P spin states. The dotted-line pointers indicatespontaneous emission. In thermal equilibrium both S states are equallypopulated and the two P states are virtually not populated at normaltemperatures. Radiating with light with a wavelength appropriate to theenergy difference between S and P state would populate both P states tosome degree. But still there would be no population difference betweenthe two S states. The situation is different if we use circularlypolarized light. This is also indicated in FIG. 6 a. Circularlypolarized light has the ability to cause transitions between the twostates connected only by the waved line in FIG. 6 a. Electrons arecontinuously pumped from the S_(−1/2) to the P_(1/2) state. From thereafter a short time they fall back to the S states or while in theP_(1/2) state they may go first into P_(−1/2) state and then fall backto the S states. Once in the S_(1/2) state they are virtually trapped,while the S_(−1/2) state is continuously depopulated by the circularlypolarized light. With this mechanism population is accumulated in theS_(1/2) state as indicated by the three little balls in FIG. 6 a

In the foregoing no magnetic field was present and the exchange betweenP_(1/2) and P_(−1/2) electron states, for example, was accomplished bycollision. If a magnetic field was present the two states would not haveequal energy and it could be necessary to radiate the transitionfrequency into the sample to facilitate the spin coupling.

FIG. 6 b shows how the electron polarization of an alkali metal such asRb is transferred to xenon nuclei via collision and spin exchange. Anoble gas like xenon is typically used to store the nuclearhyperpolarization because of its long T1 relaxation time. In thisembodiment the hyperpolarized xenon is used as the polarizing agent forthe ¹³C nuclei.

The polarizing agent such as xenon, for example, can be introduced insmall amounts into a fluid of which NMR characteristics are to bemeasured. The spin polarization of the polarizing agent can be made veryhigh by optical pumping of which details can be found in Goodson (seeabove). This spin polarization can then be transferred to a spin of anucleus of an adjacent molecule of the fluid. This spin transfer isknown under the name “Spin Polarization Induced Nuclear OverhouserEffect (SPINOE). For example, a spin transfer can occur between anucleus of xenon and a ¹³C atomic nucleus contained in sampledhydrocarbons. One advantage of the present invention is that suchtransfer thereby increases the polarization of the nuclei of the fluidmolecules far in excess of the thermal equilibrium and hence increasesthe NMR signal amplitude. The increase in NMR signal amplitude undersuch a technique can be a factor of the order of 100. Such anamplification of the signal amplitude enables a substantial reduction ofthe necessary NMR measurement time, theoretically by a factor of 10,000.

The nucleus of a polarizing agent such as a noble gas, e.g. xenon, canbe hyperpolarized and this polarization may be transferred from thehyperpolarized gas to a sample. Polarization transfer may occur using avariety of mechanisms. The transient enhancement of a signal as aconsequence of cross-relaxation and polarization transfer between thedissolved hyperpolarized gas and the surrounding solution spins is anovel manifestation of the nuclear Overhauser effect (NOE), and is knownas the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE). Adiscussion of SPINOE can be found, for example, in Goodson, “Advances inMagnetic Resonance, Nuclear Magnetic Resonance of laser-Polarized NobleGases in Molecules, Materials, and Organisms”, Journal of MagneticResonance, vol. 155, 157-216 (2002). In another mechanism, CrossPolarization (CP) locks both nuclei (noble gas and the target ofpolarization transfer) with simultaneous electromagnetic fields at twoseparate frequencies. This creates a quantum transition that enablespolarization to be efficiently transferred from one nucleus to anothernucleus.

A diagram of the process of transferring spin polarization to the samplefluids is shown in FIG. 10. An intermediate atom can be excited using avariety of methods. As shown in FIG. 10, for instance, a circularlypolarized laser beam 1001 can be shined onto an intermediate atom 1002,such as Rubidium, resulting in a excitation of an electron of theintermediate atom by bringing it from the S to the P state. Through aprocess of spin-orbit coupling, the excitation of the angular momentgives rise to a spin polarization 1003. A quenching process, using N₂,for example, leads to a spin-polarized electron in a ground state 1004.Upon contact with a polarizing agent (i.e. xenon), a spin exchangeprocess between the intermediate atom 1004 having a spin-polarizedelectron and the polarizing agent 1010 enables the transfer of spinpolarization from the electron of the intermediate atom (Rb) to thenucleus of the polarizing agent (Xe). Xenon typically has a long T₁ time(see Goodson), which is optimal for a polarizing agent. Such a transferutilizes a hyperfine interaction, and results in a polarized nuclearspin of the polarizing agent 1011. A modulated dipole-dipole interactioncan affect the nuclear spin of a hydrogen atom 1120 with which thepolarizing agent 1011 comes in contact. As the spin of the nucleus ofthe Xe atom changes polarization, the spin of the hydrogen nucleuschanges its polarization. As shown in FIG. 10, an anti-parallelalignment (a) between the polarizing agent 1011 and hydrogen nuclearspin 1120 leads to another anti-parallel alignment, with the polaritiesof the polarizing agent and the proton spin reversed (1121 and 1122,respectively). Similarly a parallel alignment (b) between the polarizingagent 1011 and proton spin 1130 leads to a parallel alignment withproton spin reversed 1132. Instead of protons, ¹³C nuclei can bepolarized in the same way, or the proton polarization may be transferredto ¹³C nuclei in a further step of SPINOE.

FIG. 11 shows a progression of the spin transfer interactions with thevarious degrees of polarization at each stage. In the instance where acircularly polarized laser beam 1101 is introduced into the system forexcitation purposes, the beam has a nearly total polarization. Upontransfer of the polarization to the intermediate stage atoms 1102,polarization can be found at ˜0.95 of the population. After the hyperfmeinteraction in which spin is transferred from an electron of theintermediate stage atom to the nucleus of the polarizing agent 1103,polarization is at 0.3 of the population. Finally, upon transfer of spinthrough the dipole-dipole interaction from the nucleus of the polarizingto the nucleus of nearby protons, due to the strength of theinteraction, polarization is at 10⁻⁵-10⁻⁴ of the population of protons1104. Further dipole-dipole interactions can be used to transfer spin toother atomic nuclei 1105.

In one mode of the present invention, hyperpolarized xenon can be usedas the nucleus for chemical shift NMR. The chemical shift range of xenonin different chemical environments is over 7000 ppm wide. While thelarge shift range results largely from strong electron deshielding inthe xenon compounds, a range of over 200 ppm may be obtained merely bydissolving xenon in various liquids. Xenon is generally chemicallyinert. However, in 1962 Neil Bartlett at the University of BritishColumbia treated xenon gas with PtF₆ and prepared the first noble gascompound consisting of platinum, fluorine and xenon. More than 80 xenoncompounds have been made with xenon chemically bonded to fluorine andoxygen. For the purposes of the present invention, the highlypolarizable electron cloud of xenon causes it to be relativelylipophilic, permitting xenon to participate in specific interactionswith various substances. This makes it possible to characterizerecovered formation fluids by performing chemical shift analysis of theNMR spectra of xenon dissolved in said formation fluids. This chemicalshift NMR is discussed below with respect to ¹³C, but the method mayalso be used with xenon NMR. In addition, xenon readily adsorbs tonumerous surfaces under experimentally convenient conditions. This,together with its lipophilic behavior enables a direct determination ofoil saturation in situ. Due to the very high hyperpolarization possiblein xenon only a trace amount would be needed for this measurement.

In one mode of the invention, the polarizing agent can be firstoptically pumped using a laser and later introduced into a chambercontaining a fluid sample to be examined using NMR techniques. The spinpolarization of the polarizing agent is transferred to the fluid, atwhich time NMR characterization can be performed.

The unpolarized agent is stored under pressure in a storage tank. Afterbeing polarized, the agent is stored again in another pressurized tankbefore being brought in contact with the fluid sample to be tested byNMR. The length of time the polarized agent can be stored depends on itsT₁ relaxation time, which for xenon is of the order of half an hour,depending on its purity and the storage vessel material.

As the T₁ of the polarizing agent can currently be increased usingprocedures discussed, for example, in Goodson, then it may be possibleto polarize the polarizing agent at the surface and transfer it to thestorage tank in the drilling tool before the NMR tool is lowered intothe borehole, thereby avoiding the need of optically pumping downhole.In wireline or coiled tubing drilling tools it may be possible to feedpolarized xenon continuously down a tube in the wireline or coiledtubing from the surface to the measurement tool downhole.

Alternatively, the polarizing agent can first be introduced into achamber containing a fluid sample to be examined using NMR techniques. Alaser beam tuned to a polarizing frequency of the polarizing agent canthen be directed into the chamber. The polarizing agent becomespolarized and then transfers its spin polarization to the nuclei of thefluids to be characterized, for instance, using the spin transfer ofSPINOE.

Chemical analysis of hydrocarbons in the formation can be performed atthe same time as other tests, i.e. formation pressure testing, withouttaking formation liquid samples to the surface.

The presence or absence of certain frequency components can be used todetermine whether aromatic hydrocarbon compounds and/or aliphatichydrocarbon compounds are present in the fluid sample. The resolution ofspin echo amplitude measurements in the method of the invention issufficient to calculate relative amplitudes of signal components at 30and 130 parts per million (ppm) from the base frequency (the frequencyof the RF power used to perform the spin echo measurement sequences.Alternatively the complete ¹³C spectrum may be obtained, e.g. bysampling FIDs and Fourier transform, or by performing CW NMR, as far asthe homogeneity and stability of the static magnetic field is enabling.

To process the digitized spin echoes into characterizing informationabout the fluid sample, each spin echo in each CPMG sequence can havetime correspondent ones of the digitized amplitude measurements summedor averaged over each entire CPMG sequence. The result of the summing isa set of digital amplitude values for each CPMG sequence. In thisembodiment of the invention, three antennas 216A, 216B, 216C areprovided at different locations along the longitudinal axis of thesensor 210. By including a plurality of antennas each energizing adifferent volume within the fluid sample, it is possible to acquire NMRsignals having improved signal-to-noise in a relatively short timeperiod. The improved signal-to-noise is obtained by summing or“stacking” the spin echoes measured using each antenna 216A, 216B, 216C.The stacking can be performed in a signal processor. The antennas 216A,216B, 216C can each be selectively energized for performing a CPMGmeasurement sequence by using the switching circuit 422. As is known inthe art, nuclei which have been transversely polarized by NMR spin echoexperimentation gradually “relax” or return to magnetic spin orientationaligned with the static magnetic field. During the longitudinalrelaxation, no further experimentation on the particular sample ispractical. The nuclei of the fluid samples in the location of thenon-energized antennas, however, remain substantially polarized alongthe static magnetic field and can be subjected to NMR spin-echoexperimentation during the longitudinal relaxation period (the “waittime”) of the previously transversely polarized (the “experimented on”)fluid sample. Spin echo amplitudes measured by each of the antennas216A, 216B, 216C can also be summed to get spin echo amplitude valueshaving improved signal-to-noise. Using three switched antennas is not alimitation on the invention, but is merely illustrative of the principleof multiple measurements made in different portions of the sample toconserve time. It is contemplated that five or more switched antennascan be used with the sensor 210 of the invention. It is furthercontemplated that two or more of the antennas can be used to conductCPMG measurements sequences simultaneously where enough such antennasare used in the particular sensor to enable sufficient wait time betweenmeasurement sequences at any single antenna. For example, a measurementcycle for a six antenna system could include measuring CPMG sequences atthe first and fourth antennas, next at the second and fifth antennas,and finally at the third and sixth antennas. The cycle can then berepeated at the first and third antennas, and so on for an appropriatenumber of cycle repetitions to obtain a sufficient signal-to-noiseratio.

A timing diagram showing typical CPMG pulse sequences applied to each ofthe antennas (216A, 216b, 216C in FIG. 4) is shown in FIG. 9. 90° and180° pulses at the 6.12 MHz resonant frequency can be applied to thefirst antenna as shown in the upper timing scale in FIG. 9. Each spinecho occurring after one of the 180° pulses is indicated by E1, E2, E3,and on through E50. Immediately after the end of the CPMG sequence at6.12 MHz at the first antenna (216A in FIG. 4) a CPMG sequence can beapplied to the second antenna (216B in FIG. 4) as shown in the secondtiming scale in FIG. 4, starting at about 510 milliseconds from theinitiation of the sensor operation. As the CPMG sequence is completed atthe second antenna, a CPMG sequence can be immediately started at thethird antenna (216C in FIG. 4). This entire sequence of CPMG sets atsuccessive antennas can be repeated as shown in the bottom timing scalein FIG. 4, representing a CPMG sequence at 6.12 MHz at the first antennastarting at about 1530 milliseconds from the start of the first suchCPMG sequence at the first antenna.

After summing, or “stacking”, the spin echo amplitude values from allthe CPMG measurement sequences, the resulting stacked spin echoamplitude sample values can then be analyzed using a fast Fouriertransform or similar spectral analysis, to generate a Fourier spectrum.The Fourier spectrum will include relative amplitude contributions ofdifferent frequency components present in the stacked spin echoamplitude values. The presence or absence of certain frequencycomponents can be used to determine whether aromatic hydrocarboncompounds and/or aliphatic hydrocarbon compounds are present in thefluid sample. The resolution of the spin echo amplitude measurements inthe method of the invention is sufficient to calculate relativeamplitudes of signal components at 30 and 130 parts per million (ppm)from the base frequency (the frequency of the RF power used to performthe spin echo measurement sequences.

For example, carbon-13 in xylene generates characteristic spectral peaksin the range of about 130 ppm from the base frequency of 6.12 MHz.Carbon-13 in typical aliphatic (alkane) compounds including CH₂ and CH₃molecular groupings therein has characteristic peaks in the 30 ppm rangefrom the base frequency. See, for example, W. Simons, The Sadtler Guideto Carbon-13 Spectra, Sadtler Research Laboratories, 1984. As is knownin the art, drilling fluids which include hydrocarbon as the liquidphase typically include aromatic compounds. Crude oils typically includesome aliphatic compounds. After performing the Fourier transform on thestacked samples, the amplitude of the spectrum at 130 ppm can bemeasured, and the amplitude of the spectrum at 30 ppm can be measured.Absence of any substantial spectral amplitude at 130 or 30 ppm indicatesthat the fluid sample does not include any substantial amount ofhydrocarbons, either aromatic or aliphatic type. If the amplitude of the130 ppm portion of the spectrum shows substantial presence of aromatichydrocarbons, and the drilling fluid contains such aromatics in theliquid phase, it may be inferred that the fluid sample includes asubstantial fraction of mud filtrate. Presence of substantial amounts ofaliphatic hydrocarbons, as indicated by substantial amplitude of the 30ppm portion of the spectrum, indicates that the fluid sample in thesensor 10 includes some connate hydrocarbons. It is therefore possibleusing the spectroscopy technique of the invention, to discriminatebetween crude oil, and oil based mud filtrate by determining therelative presence of aliphatic and aromatic compounds in the fluidsample.

When using spin echoes it is probably necessary to radiate RF directlyat the frequency of where signals are expected, e.g. at 30 ppm from basecarbon-13 resonance and in a second measurement at 130 ppm.Alternatively the interecho time needs to be specially chosen that thespin echo NMR resonance frequency, not identical with the transmitted RFfrequency, has the correct phase relationship at the position of each RFpulse. Using NMR FIDs or CWNMR instead of spin echoes avoids thisproblem.

An example of analyses using the method of the invention is shown ingraphs in FIGS. 8A-8C. FIG. 8 shows a typical analysis of a fluid samplecomprised mainly of water. Neither the 130 ppm portion of the spectrumnor the 30 ppm portion have any appreciable amplitude. In FIG. 8B, thefluid analyzed contains a substantial portion of aromatic hydrocarbon,which can be inferred from the substantial amplitude at 130 ppm and thelack of appreciable amplitude at 30 ppm. This response is typical ofoil-based mud filtrates comprised mainly of aromatic compounds. If themud filtrate is comprised of aliphatic compounds as well, the analysisof the fluid samples may be improved by first introducing a sample ofthe mud filtrate to the sensor (10 in FIG. 2) and performing NMRanalysis as described herein. The resulting analysis can be compared toanalyses made of fluids withdrawn from the earth formation to determinethe extent to which the fluid is comprised of mud filtrate. An analysisof typical crude oil sample containing both aliphatic compounds and somearomatic compounds is shown in FIG. 8C.

The discussion of ¹³C spectroscopy is an example of a best mode ofoperation of the present invention. It is not meant as a limitation ofthe invention. By changing the operating frequency of the NMR apparatus,the quantities of various isotopes can be determined. The best isotopesfor NMR measurements are ¹H, ²³Na, and ³⁵Cl. Other isotopes that can bemeasured using the techniques of the present invention include ¹⁷O,²⁵Mg, ³³S, ³⁷Cl, and ³⁹K. NMR properties of commonly occurring elementsin oilfield fluids may be found in the Table below. NMR Properties ofElements Common in Oilfield Fluids Frequency/ Natural NMR Net IsotopeFrequency (¹H) Abundance Sensitivity⁽¹⁾ Sensitivity⁽²⁾ ¹H 1 1.00 1 1 ¹³C0.251 0.011 1.59 × 10⁻² 1.75 × 10⁻⁴ ¹⁷O 0.136 3.7 × 10⁻⁴ 2.91 × 10⁻²1.08 × 10⁻⁵ ²³Na 0.264 1.00 9.25 × 10⁻² 9.25 × 10⁻² ²⁵Mg 0.061 0.1012.67 × 10⁻³  2.7 × 10⁻⁴ ³³S 0.076 0.0076 2.26 × 10⁻³ 1.72 × 10⁻⁵ ³⁵Cl0.098 0.755 4.70 × 10⁻³ 3.55 × 10⁻³ ³⁷Cl 0.082 0.245 2.71 × 10⁻³ 6.63 ×10⁻⁴ ³⁹K 0.047 0.931 5.08 × 10⁻⁴ 4.74 × 10⁻⁴⁽¹⁾At 100% abundance, ¹H = 1⁽²⁾At natural abundance, ¹H = 1

In an alternate embodiment of the invention, measurements of formationand fluid properties are made in situ, e.g., by modifying an apparatussuch as that described in U.S. Pat. No. 6,348,792, issued to Beard etal., having the same assignee as the present invention, and the contentsof which are incorporated herein by reference.

Standard methods are known in the prior art whereby in situ measurementsof T₁ and T₂ distribution enable one to determine measurement parametersof the surrounding earth formation, such as, among others, the porosityof the earth formation, permeability, and bound volume irreducible.

Permeability estimation from NMR is generally obtained using empiricalcorrelations to porosity and either a log-mean relaxation time or aNMR-derived ratio of Free/Bound Water. The bound water fraction isgenerally estimated from the inverted T₂ distribution using a sharp orgradational T₂ cutoff, based on the observation that smaller pores areassociated with shorter relaxation times.

The faster NMR relaxation in smaller pores is caused by highersurface/volume ratios, causing more frequent interactions between theproton spins and the surroundings. The permeability of a porous mediumis generally controlled by the pore throat size (capillary size), andfor sandstone the pore throat size often correlates well with pore bodysize, which again is related to grain size. The sensitivity of the NMRmeasurement to surface/volume ratio is therefore useful in predictingpermeability. Besides surface/volume ratio, the NMR relaxation ratedepends on surface type (pore wall lithology), bulk fluid properties,and diffusion relaxation caused by external or internal magneticgradients.

FIG. 12 schematically illustrates an embodiment of the present inventionwherein the shaping of the static and RF fields is accomplished in aregion within the earth formation. The tool cross-sectional view in FIG.12 illustrates a main magnet 1217, a second magnet 1218, and atransceiver antenna, comprising wires 1219 and core material 1210. Thearrows 1221 and 1223 depict the polarization (e.g., from the South poleto the North pole) of the main magnet 1217 and the secondary magnet1218. A noteworthy feature of the arrangement shown in FIG. 12 is thatthe polarization of the magnets providing the static field is towardsthe side of the tool, rather than towards the front of the tool (theright side of FIG. 12).

The second magnet 1218 is positioned to augment the shape of the staticmagnetic field by adding a second magnetic dipole in close proximity tothe RF dipole defined by the wires 1219 and the soft magnetic core 1210.This moves the center of the effective static dipole closer to the RFdipole, thereby increasing the azimuthal extent of the region ofexamination, the desirability of which has been discussed above. Thesecond magnet 1218 also reduces the shunting effect of the highpermeability magnetic core 1210 on the main magnet 1217: in the absenceof the second magnet, the DC field would be effectively shorted by thecore 1210. Thus, the second magnet, besides acting as a shaping magnetfor shaping the static field to the front of the tool (the side of themain magnet) also acts as a bucking magnet with respect to the staticfield in the core 1210. Those versed in the art would recognize that thebucking function and a limited shaping could be accomplished simply byhaving a gap in the core; however, since some kind of field shaping isrequired on the front side of the tool, in an embodiment of theinvention, the second magnet serves both for field shaping and forbucking. If the static field in the core 1210 is close to zero, then themagnetostrictive ringing from the core is substantially eliminated.

Within the region of investigation, the static field gradient issubstantially uniform and the static field strength lies withinpredetermined limits to give a substantially uniform Larmor frequency.Those versed in the art would recognize that the combination of fieldshaping and bucking could be accomplished by other magnet configurationsthan those shown in FIG. 12. For example, FIG. 12A shows a single magnet1227 and magnetic core 1230 that produces substantially the same staticfield as that produced by the combination of magnets 1217 and 1218 inFIG. 12. A substantially similar field configuration results from thearrangement in FIG. 12B with the magnet 1237 and the core 1240. What isbeing accomplished by the magnet arrangements in FIGS. 12, 12A and 12Bis an asymmetry in the static magnetic field in a direction orthogonalto the direction of magnetization. In an optional embodiment of theinvention (not shown) the second magnet is omitted.

Returning to FIG. 12, the transceiver wires 1219 and core pieces 1210should be separated as far as possible towards the sides of the tool.This separation increases the transceiver antenna efficiency byincreasing the effective RF dipole of the antenna and augments the shapeof the RF magnetic field isolines so that they better conform to thestatic magnetic field isolines. The secondary magnet is made of amaterial such, as a nonconducting material, which minimizes eddycurrents induced by the RF field, thereby increasing the RF antennaefficiency.

The NMR tool described above with reference to FIG. 12 is an example ofa tool that may be used for determining formation properties. Many othersuitable arrangements of magnets and antennae may be used. Those versedin the art would recognize that NMR sensors can utilize the earthmagnetic field to perform a measurement. In this case no permanentmagnets are required in the tool. It should also be noted that theinvention may also be practice when the downhole tool is conveyed on awireline.

While the foregoing disclosure is directed to the preferred embodimentsof the invention, various modifications will be apparent to thoseskilled in the art. It is intended that all such variations within thescope and spirit of the appended claims be embraced by the foregoingdisclosure.

1-26. (canceled)
 27. A method of obtaining a parameter of interest of anearth formation, comprising: (a) using a magnet on a nuclear magneticresonance (NMR) sensor of a downhole logging tool for aligning spins ofnuclei in a region of interest of said earth formation; (b) polarizingnuclear spins of a polarizing agent carried in a chamber on said loggingtool; (c) introducing said polarizing agent into said earth formationand enhancing alignment of spins of said nuclei in said region ofinterest; (d) applying a radio frequency (RF) pulse sequence to saidearth formation with at least one transmitter on said NMR sensor; and(e) obtaining NMR signals from said region of interest in response tosaid radio frequency pulse sequence at at least one receiver antenna.28. The method of claim 27 wherein said obtained NMR signals comprise afree induction decay.
 29. The method of claim 27 wherein said obtainedNMR signals comprise spin echo signals.
 30. The method of claim 29wherein said RF pulse sequence comprises an excitation pulse and aplurality of refocusing pulses.
 31. The method of claim 30 wherein saidexcitation pulse has a tip angle of substantially equal to 90°.
 32. Themethod of claim 30 wherein said plurality of refocusing pulses have tipangles substantially equal to 180°.
 33. The method of claim 30 whereinsaid plurality of refocusing pulses have tip angles between 90° and180°.
 34. The method of claim 29 further comprising using a processorassociated with said logging tool for obtaining a longitudinalrelaxation time (T₁) distribution of said earth formation.
 35. Themethod of claim 29 further comprising using a processor associated withsaid logging tool for obtaining a transverse relaxation time (T₂)distribution of said earth formation
 36. The method of claim 29 whereinsaid parameter of interest is at least one of (i) porosity, (ii) claybound water, (iii) bound volume irreducible, and, (iv) permeability. 37.The method of claim 27 wherein said polarizing agent comprises a noblegas.
 38. The method of claim 27 wherein said noble gas comprises Xenon.39. The method of claim 27 wherein polarizing said nuclear spins of saidpolarizing agent further comprises a spin exchange with an intermediatematerial.
 40. The method of claim 39 wherein said intermediate materialcomprises rubidium.
 41. The method of claim 39 further comprisingirradiating said intermediate material with a laser to move electrons ofsaid intermediate material to a higher quantum state. 42-63. (canceled)64. An apparatus for obtaining a parameter of interest of an earthformation, comprising: (a) a magnet on a nuclear magnetic resonance(NMR) sensor of a downhole logging tool which aligns spins of nuclei ina region of interest of said earth formation; (b) a chamber on saidlogging tool, the chamber containing a polarizing agent; (c) a devicewhich polarizes spins of said polarizing agent and conveys saidpolarizing agent into said earth formation thereby enhancing alignmentof spins of said nuclei in said region of interest; (d) a transmitterwhich applies a radio frequency (RF) pulse sequence to said earthformation; (e) a receiver which receives NMR signals from said region ofinterest responsive to said radio frequency pulse ; and (f) a processorwhich determines from said NMR signals a parameter of interest of saidearth formation.
 65. The apparatus of claim 64 wherein said obtained NMRsignals comprise a free induction decay.
 66. The apparatus of claim 65wherein said obtained NMR signals comprise spin echo signals
 67. Theapparatus of claim 66 wherein said RF pulse sequence comprises anexcitation pulse and a plurality of refocusing pulses.
 68. The apparatusof claim 67 wherein said excitation pulse has a tip angle ofsubstantially equal to 90°.
 69. The apparatus of claim 64 wherein saidprocessor obtains a longitudinal relaxation time (T₁) distribution timeof said earth formation.
 70. The apparatus of claim 64 wherein saidparameter of interest is at least one of (i) porosity, (ii) clay boundwater, (iii) bound volume irreducible, and, (iv) permeability.
 71. Theapparatus of claim 64 wherein said polarizing agent comprises a noblegas.
 72. The apparatus of claim 71 wherein said noble gas comprisesxenon.
 73. The apparatus of claim 64 wherein the device polarizes saidnuclear spins of said polarizing agent by further enbaling a spinexchange with an intermediate material.
 74. The apparatus of claim 73wherein said intermediate material comprises rubidium.
 75. The apparatusof claim 73 further comprising a laser which irradiates saidintermediate material to cause transitions from the S to the P quantumstate of electrons of said intermediate material. 76-80. (canceled) 81.A method of using a logging tool for analyzing a fluid of an earthformation, the method comprising: (a) dissolving a polarizing agent intosaid fluid; (b) using an NMR sensor on said logging tool for obtainingNMR signals from said dissolved polarizing agent.
 82. The method ofclaim 81 wherein said dissolving of said polarizing agent is done in theearth formation.
 83. The method of claim 81 wherein said dissolving ofsaid polarizing agent is done in a fluid sample chamber on said loggingtool, the method further comprising recovering said formation fluid fromsaid earth formation using a fluid sampling device on said logging tool.84. The method of claim 81 wherein said NMR signals correspond to freeinduction decay of a nucleus of said polarizing agent.
 85. The method ofclaim 84 further comprising chemical shift NMR analysis of said NMRsignals.
 86. The method of claim 81 where said NMR signals comprise of aCW frequency spectrum to obtain chemical shift information.